[whatwg] Canvas Proposal: aliasClipping property

On Fri, Oct 16, 2009 at 2:25 PM, Robert O'Callahan <robert at ocallahan.org> wrote:
> On Sat, Oct 17, 2009 at 1:06 AM, Philip Taylor <excors+whatwg at gmail.com>
> wrote:
>> I think the spec is clear on this (at least when I last looked; not
>> sure if it's changed since then). Image A is infinite and filled with
>> transparent black, then you draw the shape onto it (with no
>> compositing yet), and then you composite the whole of image A (using
>> globalCompositeOperation) on top of the current canvas bitmap. With
>> some composite operations that's a different result than if you only
>> composited pixels within the extent of the shapes you drew onto image
>> A.
> Ah, so you mean Firefox is right in this case?

Yes, mostly. http://philip.html5.org/tests/canvas/suite/tests/index.2d.composite.uncovered.html
has relevant tests, matching what I believed the spec said - on
Windows, Opera 10 passes them all, Firefox 3.5 passes all except
'copy' (https://bugzilla.mozilla.org/show_bug.cgi?id=366283), Safari 4
and Chrome 3 fail them all.

(Looking at the spec quickly now, I don't see anything that actually
states this explicitly - the only reference to infinite transparent
black bitmaps is when drawing shadows. But
is phrased in terms of rendering shapes onto an image, then
compositing the image within the clipping region, so I believe it is
meant to work as I said (and definitely not by compositing only within
the extent of the shape drawn onto the image).)

Philip Taylor
excors at gmail.com

Received on Friday, 16 October 2009 08:01:41 UTC